1. Field of the Invention
The present invention relates to a gap measuring device and a gap measuring method for measuring a narrow gap utilizing the tunnel effect of light wave reflected at a boundary plane on the condition of total reflection.
2. Description of the Prior Art
High-precision laser range finders such as Michelson interferometers are used for precise measurement of distances utilizing the interference of laser beams. The high-precision laser range finders determine a distance based on the principle that the number of interference fringes formed due to the interference of laser beams is directly proportional to a displacement of a movable mirror (or a rectangular prism).
High-precision laser range finder are, however, not suitable for measuring a gap between a mask and a wafer or for measuring unevenness of the wafer in the printing process, i.e., in printing a mask pattern onto the wafer in semiconductor manufacture. The high-precision laser range finders require movement of the movable mirror in response to the dimension of the gap between the mask and the wafer or to the degree of the wafer unevenness; this is, however, impossible when the dimension of the gap or the unevenness is unknown.
Thus, there are substantially no apparatus for precisely measuring a narrow gap or unevenness on a surface.